1. Field of the Invention
The present invention relates to image processing techniques for generating halftone spot images in order to make continuous-tone prints like photographs. More in particularly, this invention relates to image processing techniques for setting screen angles determined by conversion tables used for the generation of halftone spot images in such a manner as to weaken the effect of moire.
2. Description of the Related Art
An electrophotography method is known as an image recording method used for color printers, color copying machines, and the like. In such an electrophotography method, laser beams are utilized to form latent images on a photoconductor drum, the latent images are developed by using charged toner, and developed images formed with the toner are transferred and fixed on transfer paper, thereby recording the images.
Such color printers and the like record color images generally by utilizing toner in cyan (C), magenta (M), yellow (Y), and black (K). The respective colors which are expressed in intermediate tones are mixed by means of subtractive color mixture, thereby generating record images.
As an intermediate tone expression method, a dither method is known in which a pseudo intermediate tone expression is realized by using binary recording. The dither method is a method of obtaining binary data through conversion from tone data at each dot by using a threshold value that is changeable for every dot, and dots in an original image are in a one-to-one correspondence with dots in a record image. A general way to determine a threshold value for each dot is to consider an original image as a set of submatrices and to decide a threshold value only according to coordinate information in the relevant submatrix. The submatrices with such threshold values located therein are called dither matrices. Since the dither matrices are referred to when the tone data of the original images are converted into the gradation expression of the record images, the dither matrices can be considered as a kind of conversion tables from the original images to the record images.
There are some kinds of conversion tables like the dither matrices, such as a dot dispersion type and a dot concentration type. In the case of the dot concentration type, a plurality of adjacent dots compose a halftone spot, and intermediate tones are expressed according to the size of this halftone spot. In this specification of the present invention, record images of which intermediate tones are expressed with halftone spots in the manner described above are called halftone spot images.